Short answer: You don’t need to.
Long answer: There’s little need to convert monsters from Dungeons & Dragons to Mutants & Masterminds simply because 95% of the stat block information is the same, so just use it as-is, right off the page. Where there are differences in game terms (most notably the skill list), just use the one that you’re given. Work out the Toughness, Defense and Damage ratings, and you’re good to go.
For example, if you’re using a monster from 3rd Edition D&D you’ve got skills such as Spot, Move Silently and Search. Don’t sweat it, and just use them rather than M&M’s Stealth and Notice. Zero conversion required. Don’t worry about breaking down the critter’s abilities into M&M-compliant Powers or bother about the points costs – just use them as they’re written, and enjoy. If it’s a monster you really, really want in 100% Mutants & Masterminds format (for example, if it’s going to be a Player Character, or you want to modify it’s abilities in some way), then go right ahead, but zero-conversion will work just fine for 99% of your needs, and why make work for yourself when the poor beastie is (hopefully) going to be dead in under 10 rounds?
M&M and D&D (both 3rd and 4th Edition) overlap where it comes to stats, ability bonuses, skills, feats (mostly – again, don’t sweat it), attack bonuses and saves, so rather than try to convert like-to-like, just use what you’re given. They’ll be good enough for the majority of your needs. Honest.
The main difference lays in the way both systems give and receive damage. D&D has Hit Points, AC and funky dice rolls, whereas as M&M uses Toughness Saves, Defense and a damage track. Thankfully, converting these numbers is simple enough to do right at the table during play.
Dishing out damage
Use the attack roll as given (ie, if the monster has +2 to hit with their Axe, then that’s still +2 to hit.
For damage, half the maximum dice roll, -1, then add any bonuses due to Strength, magic, etc.
This gives you the attack’s Damage Bonus. For example, a Longsword (1d8) has a Damage Bonus of (8/2-1) 3 (ie, require a DC 18 Toughness save). In the hands of a STR 16 Gnoll it would have a Damage Bonus of 6 (DC 21). If he had a Longsword +1, the Damage Bonus would be 7 (DC 22).
This works just the same for 4e conversions. Use the same to-hit roll, and just calculate the Damage Bonus for each Power, and you’re done.
Instead of Armour Class and Hit Points, M&M uses Defense and a Toughness save; attacks are rolled against Defense, then there’s a Toughness save against the Damage DC. Fail the save and (depending on how much you fail by), you’re Bruised, Staggered, or worse.
Defense is similar to Armour Class; it’s not quite the same as some things which add to AC in D&D instead modify your Defense in M&M. Most notably, in M&M armour adds to your Toughness Save, not your Defense. Shields, on the other hand, do add to Defense. This is more realistic – wearing armour doesn’t make you harder to hit, but it does mean that a hit is less likely to injure you, and shields are used to deflect attacks.
Make Defense equal to AC minus Armour bonus.
To calculate Toughness save, use CON bonus + number of Hit Die (or Level, for 4e) + Armour bonus. Eyeball the result; if the monster’s Toughness is too high or low, adjust to taste.
Let’s pick on the poor 3e Goblin straight from the SRD. Here’s the relevant game stats with the numbers we can use directly in our M&M game highlighted.
Goblin, 1st-Level Warrior
Small Humanoid (Goblinoid)
Str 11, Dex 13, Con 12, Int 10, Wis 9, Cha 6
AC 15 (+1 size, +1 Dex, +2 leather armour, +1 light shield)
HD 1d8+1 (5 hp), +1 Init, Speed 30′
Fort +3, Ref +1, Will -1
Morningstar +2 (1d6) or javelin +3 (1d4)
Hide +5, Listen +2, Move Silently +5, Ride +4, Spot +2
Alertness, Darkvision 60 ft.
That Morningstar’s Damage Bonus is going to be (6/2-1 = 2), making it DC 17, and the Javelin is (4/2-1= 1) DC 16. Defense is (AC 15 – 2) = 13, and Toughness save is (1 +2) +3. Done.
We’ve one Goblin ready to roll! If we want to make him a Goblin Minion, just note the fact. In M&M this means that one failed Toughness save and he’s out of action. Simple, really.
More complex monsters might take slightly more time (due to having more attack options), but the principle is still the same – use what’s there, and just calculate the rest as needed. Why work out the DCs for every single spell and attack if they’re not going to be used. Work it out on the fly instead.
Skills, Stats, Saves and Attack rolls: stay the same
Damage: half the maximum dice roll, -1, then add any bonuses due to Strength, magic, etc.
Defense = AC – Armour bonus
Toughness = 10+CON bonus + number of Hit Die (or Level, for 4e) + Armour bonus
Till next time!